Approximately one-in-four of the agencies Kelly surveyed have
executed what is known as a "tower dump," which involves
collecting phone records from wireless providers that show the
identity, activity and location of any phone that's been picked
up by a set of cell towers over a given time frame.

And at least 25 police departments own Stingrays — suitcase-sized
devices that Kelly describes as "fake cell towers." They can be
clandestinely driven into neighborhoods and "trick all nearby
phones into connecting to it and feeding data to police." Some
states allow any local department to use them, and the purchases,
which cost up to $400,000, are usually funded by the government
through anti-terror grants.

Both tactics are legal, and all of the instances Kelly cites were
predicated on obtaining warrants or court orders, or occurred in
states with no statutes governing cell phone data privacy.

But Kelly reports it's not clear how long the data is retained,
and the secrecy with which makes it difficult to exact oversight
over the departments.

"Brian Owsley, a former magistrate who reviewed many police
requests for bulk cellphone data, grew skeptical because
authorities were not always forthcoming about the technology or
what happened with 'collateral data' of innocent bystanders.
'What is the government doing with the data?" asks Owsley."